1. Field of the Invention
This invention resides in the field of transition metal suicides and methods for enhancing the low-temperature ductility of these materials.
2. Description of the Prior Art
Silicides of transition metals are useful structural materials for electronic devices where they are used as contacts and interconnects, and for larger scale equipment intended for use in oxidizing environments at high temperatures. Examples of non-electronic devices in which transition metal silicides are used are furnace heating elements, molten metal lances, industrial gas burners, aerospace turbine engine components, diesel engine glow plugs, and glass processing equipment. Molybdenum disilicide is of particular interest among transition metal silicides due to its high melting point, low density, high oxidation resistance, high thermal and electrical conductivity, and compatibility with ceramic reinforcement phases, although other transition metal silicides have similar properties.
Despite their beneficial features, the usefulness of transition metal suicides is limited by their low ductility (lack of fracture toughness) at low temperatures, their low strength at high temperatures, and a tendency toward pesting, i.e., a disintegration into powder that is thought to be the result of accelerated oxidation. Fracture toughness at low temperatures is a particular problem. The low-temperature fracture toughness of using molybdenum silicide, for example, 3 MPaxc2x7mxc2xd as compared to a required minimum of about 10 MPaxc2x7mxc2xd for industrial applications and about 15-20 MPaxc2x7Mxc2xd for turbines. The low fracture toughness also makes molybdenum disilicide difficult to machine, and effective machining is achievable only by the costly method of electro-discharge machining.
Attempts to enhance the plasticity and thereby improve the fracture toughness of molybdenum disilicide and other transition metal silicides have included pre-straining of the material at high temperature, applying surface coatings (an example of which is zirconia), and forming composites by the inclusion of a second phase such as ceramic and metallic fibers or particles. Some of these methods are disclosed by Petrovic, J. J., xe2x80x9cToughening Strategies for MoSi2-Based High Temperature Structural Silicides,xe2x80x9d Intermelallics 8: 1175-1182 (2000), and Gibala, R., et al., xe2x80x9cPlasticity Enhancement Mechanisms in MoSi2,xe2x80x9d Mater. Sci. Eng. A261:122-130 (1999). All literature and patent citations throughout this specification are incorporated herein by reference.
While composites may improve the fracture toughness, the synthesis and processing of composites are often difficult and expensive. An alternative is the formation of alloys by the incorporation of alloying elements. A variety of alloying elements have been proposed, including elements that serve as substitutes on the transition metal sub-lattice and those that serve as substitutes on the silicon sub-lattice. The most promising alloying element to date, in view of its high disembrittlement parameter, is magnesium, according to the studies of Waghmare, U.V., et al., as reported in xe2x80x9cMicroalloying for Ductility in Molybdenum Disilicide,xe2x80x9d Mater. Sci. Engin. A261: 147-157 (1999). Magnesium has a high volatility, however, which renders conventional alloying methods such as arc melting unsuitable.
It has now been discovered that a transition metal silicide alloy containing one or more alloying elements that provide the alloy with a greater fracture toughness than that of the transition metal silicide itself can be formed by combining elemental powders of the metals into a mixture and subjecting the mixture to mechanical activation followed by densification and field-activated reaction. The mechanical activation is achieved by milling and causes the alloying element to chemically combine with the transition metal by incorporation into the transition metal crystal structure. The densification and field-activated reaction are then performed by applying a compressive force to the transition metal (with the alloying element incorporated therein) and the silicon while exposing the materials to an electric current at a sufficient intensity and for a sufficient time to cause formation of the transition metal silicide in a crystal lattice that incorporates the alloying element into the lattice structure.
The method of the present invention minimizes the presence of secondary phases and produces an alloy whose microstructure consists mostly if not entirely of a single crystalline phase. These and other features, objects and advantages of the invention are explained in detail below.